By fusing human cells to mouse cells carrying an integrated sarcoma virus genome, a hybrid cell can be made that will be of potential use for biologic detection of virus particles. When the hybrid cell is infected with a replicating type C virus (leukemia virus) the sarcoma virus can be "rescued" and transforms the cells' morphology yielding a scorable "focus or plaque". Because of the human genes present human derived virus may be introduced into such a hybrid, and this cell can be used for focus formation assay.